Steam drive with viscous volatile buffer



Oct. 17, 1967 s. MATTHEWS ET AL 3,347,313

STEAM DRIVE WITH VISCOUS VOLATILE BUFFER FiledNOV. 13, 1964 .rx-wmi EmINVENTORSI C. S. MATTHEWS R. D. SEBA v THEIR ATTORNEY United StatesPatent 3,347,313 STEAM DRIVE WITH VISCOUS VOLATILE BUFFER Charles S.Matthews, Houston, and Richard D. Seba, Midland, Tex., assignors toShell Oil Company, New York, N.Y., a corporation of Delaware Filed Nov.13, 1964, Ser. No. 410,956 6 Claims. (Cl. 166-11) This invention relatesto an improved method of recovering petroleum from subterraneanreservoir formations using steam injection, and more particularly, to amethod involving the use of a steam-distillable buffer to isolate theinjected steam for the reservoir petroleum.

It is well known that the primary recovery of petroleum, methods usingthe natural flow of the petroleum, can recover only a small portion ofthe petroleum actually in the reservoir. In reservoir formations wherenatural flow will occur only about 30% or less of the actual petroleumin the reservoir is recovered. To recover additional petroleum from suchreservoir formations supplementary techniques must be used which arecommonly referred to as secondary recovery methods. Of course, there arealso reservoirs where the petroleum is too viscous to flow naturally andsupplementary techniques must be employed initially to recover anypetroleum, e.g., the tar sands. In both of these situations since thepetroleum will not flow naturally it must be displaced from theformation, or the natural conditions modified so the petroleum willflow. Thus, these supplementary techniques usually employ either adisplacing fluid which is forced into the reservoir to displace thepetroleum or add thermal energy to the reservoir so the petroleum willflow out of the reservoir.

Conventional displacing fluids are water, gases such as hydrocarbons,hydrocaron liquids and steam. Applications of these various displacingfluids have strikingly different results. For example, when usinggaseous displacing fluids the ultimate displacement of petroleum fromthe reservoir is increased approximately 20 of the original petroleum inthe reservoir and when water is used as the displacing fluid thepetroleum recovery can be increased by 50% of the original petroleum inthe reservoir. Further displacement increases can be accomplished withwater floods if the Water displacing medium has viscosity-increasingagents contained therein.

These findings support the studies which show that the efliciency ofinjected fluids in displacing petroleum from reservoir formations isdirectly related to the relative viscosities, and more properly themobility ratio of the displacing and the displaced petroleum. This ismost noticeable with respect to very viscous crudes, but remains asignificant factor in virtually all supplementary recovery methodsinvolving injected fluids to displace the residual petroleum from thereservoir.

This relationship can be expressed mathematically by the followingequation which shows the behavior of fluids flowing through porousmedia, such as petroleum-bearing reservoirs:

wherein M is the mobility of j the petroleum to the reservoir inquestion M is the mobility of the flooding medium to the reservoir inquestion 0 is the viscosity of the driven petroleum ,ue is the viscosityof the flooding medium K is the relative permeability of the reservoirtoward the flooding medium in the presence of residual petroleum3,347,313 Patented Oct. 17, 1967 K is the relative permeability of thereservoir toward the petroleum in the presence of connate water.

It is apparent from the equation that when the displacing fluid has thesame viscosity as the displaced petroleum, the mobility ratio willapproach unit and injected fluid will tend to push the petroleum throughthe reservoir ahead of it. Thus, the displacing fluid will tend to drivethe petroleum from the reservoir ahead of it as the former is injected.If the mobility ratio is approximately equal to unity, a markedlygreater volume of petroleum will be displaced by the displacing fluidinjected into the formation. Highly viscous displacing fluids give thedesired results but they are often expensive and require considerablepressures to force them through the formation which may result inunwanted fractures or thief zones which offset their advantages.

When using liquid displacing fluids, such as water, it is often thepractice to add agents to increase the viscosity of the water to obtaina better mobility ratio and a more eflicient displacement. However,there is no ready solution on how to obtain improved mobility ratio whengases are used as the displacing fluid. Steam injection does lower theviscosities of the petroleum but, unless it vaporizes the petroleum, themobility ratio is still unfavorable, even though additional petroleum isrecovered.

The general problem experienced when the mobility ratio is less than oneis that the displacing fluid tends to bypass substantial portions of thepetroleum in the reservoir since the displacing fluid has a greatermobility in the reservoir and seeks paths or channels where there is theleast resistance. Thus, when gaseous displacing fluids are used themobility ratio is unfavorable since the gases are highly mobile in thereservoir when compared to the petroleum. When petroleum is viscous,displacement via gaseous displacing fluids is generally unsatisfactory.

When steam is used as a displacing fluid, especially for viscouspetroleum crudes, much of the petroleum is bypassed even though the heatin steam lowers the viscosity of the petroleum and may distill offportions of it. While some recovery is effected by these factors,considerable petroleum remains in the reservoir, even after several porevolumes of steam have been injected. Since the steam is highly mobilerelative to liquids in the reservoir, it will have an unfavorablemobility ratio, hence a low displacement efllciency.

Another problem with steam injection is that the steam contacting thereservoir petroleum tends to drive off the lighter hydrocarbon fractionsdissolved in the petroleum leaving a more viscous petroleum residuewhich is very difficult to displace from the reservoir formation. Thisis especially true if the reservoir contains viscous crudes.

The purpose of the present invention is to provide a method by which thedisplacement efliciency of injected steam can be improved, and a greaterpercentage of pe troleum can be recovered. This is done by injecting, asa slug ahead of the steam, a viscous, steam-distillable liquid. In thisway a high displacement efliciency will be achieved by the slug and theslug will be moved through the formation by steam distillation.

In its broadest aspect this invention contemplates the method ofachieving efficient petroleum recovery from reservoirs penetrated by atleast one injection well and at least one production well by firstinjecting through the injection well a slug of a viscous,steam-distillable liquid, subsequently injecting steam to force the slugand residual petroleum into the area of the production well, andcollecting the petroleum from the production well.

Illustrated in the single accompanying figure is a vertical section of aformation having a petroleum producing reservoir which is penetrated bytwo spaced well bores and showing the reservoir divided into five zonesto diagrammatically show the conditions in the reservoir at anintermediate point during the practice of this invention.

It is through the use of viscous, steam-distillable buffer sluginterposed between the reservoir petroleum and the injected steam thatthis invention can accomplish 'high displacement effi ciencies. This ispossible since the mobility of the viscous slug is approximately thesame as that of the petroleum, or put another way, their mobility ratiois almost unity. Because the slug is injected ahead of the steam, itisolates the petroleum from the hot steam and the slug eflicientlydisplaces the petroleum as it moves through the reservoir, driven by thesteam. The invention requires that the slug be steam-distillable, and itcan be appreciated that when the steam is injected behind the slug itwill flash-oft (vaporize) the trailing edge of the viscous slug. Whenthe trailing edge of the slug is vaporized, this vaporized slug portionwill have high mobility and return to the slug proper releasing itslatent heat when it recondenses in the cooler slug. It is through thetechnique of establishing two zones of where the mobility ratios arefavorable for eflicient displacement that this invention is able toaccomplish superior displacement of petroleum with steam drives.

A greater understanding of the invention can be had by referring to theaccompanying drawing showing a geological formation penetrated by twospaced wells. For convenience, one will be referred to as injection well10 and the other will be referred to as a production well 11. It shouldbe appreciated that multiple injection wells and production wells couldbe used but it is not necessary to illustrate additional wells todescribe the invention. Normally, the injection well 10 would becircumferentially ringed by production well 11, such as the 5 or 7 spotpatterns, or some similar arrangement giving a like effect.

In the .drawing the injection well 10 is identical to production well 11and they are spaced from one another so that fluids injected through theinjection well 10 will pass through the formation to the production well11 displacing the petroleum in the reservoir formation ahead of theinjected fluids.

The borehole of both wells 10 and 11 have casing strings 12 which extenddownwardly through a multilayered formation 13 and penetratepetroleum-producing reservoir 14. Reservoir 14 is shown sandwichedbetween overburden and underburden strata 13 having impermeable layerscontiguous to it. Casing strings 12 are sealed in the reservoir at leastto the extent of their vertical traverse of the reservoir 14 with asealant 15 such as cement, and the strings and sealant are perforatedwith ports 16 in the area of the reservoir 14 to provide fluidcommunication between the inside of the casing strings 12 and thereservoir proper.

Injection well 10 and production well 11 both have pipe strings 17extending downwardly into the casing strings 12 to the vicinity ofreservoir 14 and the annulus between the pipe strings and theirrespective casing strings is sealed near the top of the reservoir withpackers 18. It is preferable that the pipe strings 17 extend to thebottoms 19 of the respective casing strings and this is desirable in theproduction well 11 so the petroleum can be easily recovered therefrom.Generally, the pipe strings 17 will be secured to the top of the casingstrings 12 at the wellhead with a gland 20 or some similar typearrangement.

In the practice of the invention a slug of a viscous, steam-distillableliquid will be injected through injection well 10 and will, as indicatedby arrows 21, travel down pipe string 17 into the lower portion ofcasing string 12, from which it moves out into the formation throughports 16. After the buffer slug has been injected it is followed by theinjection of steam which follows the same path as the slug.

The reservoir 14 between the injection well 10 and production well 11 isshown divided into five zones so that invention can be betterunderstood. It is a representation of conditions after the slug has beeninjected and short- 1y after the injection of steam has been commenced.Zone A represents a portion of the reservoir which has been traversed bythe buffer slug, shown located in Zone C, and which is largely filledwith steam, having been stripped of petroleum by the passing of theslug. Zone B represents the steam buffer interface and it is at thisinterface where the hot steam distills the trailing edge of the buffer,converting it to a vapor which recondenses on the buffer side of theinterface, giving up its latent heat to the slug. Since the steam andthe vaporized buifer are both vapors they have a mobility ratioapproximating unity allowing both to move through the formation withequal ease.

The main portion of the buffer is shown in Zone C and because of itshigh viscosity and miscibility is able to force the petroleum ahead ofit in a bank shown as Zone D. There is an excellent mobility ratio atthe buffer petroleum interface as the buffer is pushed through thereservoir by the steam which accounts for efficient displacement of theresidual petroleum of this invention. The petroleum bank in Zone D willdrain into the production well 11 through ports 16 and be recovered asindicated by arrows 22. Zone E merely represents the reservoir in itsnatural state or water flood residual.

It should be appreciated that the buffer, which may be injected hot orcold, is actually heated by the steam and the latent heat of vaporizedportions of the buffer when it condenses therein. This causes the bufferto act as a heat source for the petroleum that it is displacing, whichwill lower petroleums viscosity making displacement easier, therebyenhancing recovery.

By using the steam-distillable bufler slug it is possible to achieve thedesirable mobility ratios in the reservoir system when using steam sothat use of the steams thermal energy in the reservoir accomplishedwithout sacrifice of efficient displacement of the petroleum in thereservoir.

Obviously, the buffer slug acceptable in the practice of this inventionmust meet certain specific characteristics. The buffer will be a liquidand should have a viscosity close to that of the petroleum to bedisplaced. It is preferred that its viscosity be equal to or be greaterthan the petroleum to be displaced. In the case of very viscous crudesthis will not be possible. The reason for this requirement is to achievethe optimum displacement of the petroleum by obtaining optimum mobilityratios.

Next, the buffer must be steam-distillable since this is imperative inthe practice of this invention. It is immaterial if the viscous bufferis a single compound or a mixture as long as the buffer is more volatilethan the reservoir petroleum. Further, the buffer must be miscible withthe petroleum in the reservoir. When the viscosity of the buffer isabout the same as the petroleum in the reservoir there will be littlemixing of the buffer and the petroleum even though they are miscible.

Some specific buffers which are useful in the practice of this inventionare alcohols, organic acids, esters, amides, etc., having a viscosity atleast substantially equal to the viscosity of the crude oil and it wasfound that size of the butter slug injected ahead of the steam couldvary from 5% to 10% of the petroleum in the reservoir by volume betweenthe injection well and the production well(s). Slugs about 1% of thepetroleum to be displaced improved the recovery only slightly whileslugs between 5% and 10% demonstrated substantial improvement.

Another advantage of the invention is that the steam injection can becontinued until the slug itself is recovered from the reservoir and thebuffer can thereafter be recycled in another reservoir. However, it maybe necessary to regenerate the buffer before it is recycled.

The steam injected may be either saturated or unsaturated and the onlyimportant criteria is that it have sufficient thermal energy to steamdistill the trailing edge of the buffer slug.

In the specification a viscous buffer slug has been referred to and thisterminology is used relative to the viscosity of the petroleum to bedisplaced by the slug. Actually, it is desirable that the slug have aviscosity close to that of the petroleum, that is not less than 75% ofthe viscosity of the petroleum to be displaced, and preferably about thesame as the viscosity of the petroleum. A disadvantage of addition of alight volatile petroleum hydrocarbon such as butane, pentane, hexane ora mixture of light hydrocarbons is that these more volatile hydrocarbonswill also be much less viscous than the heavy viscous hydrocarbons to bedisplaced, lowering the displacement efiiciencies. This will lead tofingering of the volatile solvent through the viscous hydrocarbons andthe loss of the volatile slug, and its corresponding beneficial eifecton displacement. It should be appreciated that this is a matter ofdegree and not principle.

The following experiments were conducted to illustrate the invention andare not intended to place any limitation thereon:

Example I Unconsolidated sand packs having a permeability of 25 darcies,a porosity of 36% and an initial water saturation of 22% were prepared.The test sand packs were confined in a container and saturated withnormal hexadecane to simulate reservoir conditions. Octyl alcohol wasused as the slug material and three dilferent size slugs were used inthree separate test packs. Slugs of 1%, 5% and of the hexadecanesaturation by volume of octyl alcohol were injected into these sandpacks. In each case the buffer slug was followed by steam at 30 p.s.i.g.and a temperature of 274 F.

In the pack having a 1% buffer slug no appreciable improvement over theuse of steam alone was observed. Using the 5% buffer slug it was foundthat substantial improvement in the recovery of hexadecane was observedwhile recovery of the hexadecane in the pack having the 10% buffer slugwas not noticeably better than that of the pack with the 5% buffer slug.

Example 11 A sand pack identical to those described above was saturatedwith normal hexadecane also and a mineral oil having a viscosityapproximately that of the octyl alcohol was used for the slug. This slugwas a 5% slug and was not steam-distillable. It was followed by steaminjected at the same temperature and pressure as in Example I.

Data showed this slug breakthrough into the recovery zone after about70% of the hexadecane had been recovered. When steam-distillable octylalcohol was used under identical conditions (Example I) breakthrough ofthe slug did not occur until almost 95% of the hexadecane was swept fromthe sand pack.

We claim as our invention:

1. A method of recovering petroleum from subterranean reservoirs usingthe injection of steam in which high displacement elficiencies of thepetroleum are achieved comprising:

(a). penetrating a petroleum-producing reservoir with at least oneinjection well and at least one production well spaced therefrom;

(b) establishing fluid communication between each of said wells and saidpetroleum reservoir;

(c) injecting through said injection well a slug of a steam-distillablebuffer which is a viscous liquid miscible with and more volatile thanthe petroleum in the reservoir having a viscosity of not less than ofthe petroleum in the reservoir;

((1) subsequently injecting through said injection well steam at atemperature suflicient to steam distill at least the trailing edge ofsaid steam-distillable buffer; and

(e) recovering displaced petroleum from said reservoir through saidproduction well as it is flowed into said well by said injection ofsteam.

2. A method according to claim 1 in which the slug of thesteam-distillable butter is at least equal to 5% by volume of thepetroleum to be displaced from the reservoir.

3. A method according to claim 1 in which the steamdistillable buiferhas a viscosity essentially the same as the viscosity of the petroleumto be displaced and a boiling point below that of the petroleum.

4. A method according to claim 1 in which the steamdistillable buffer isa constant boiling azeotrope having a viscosity essentially the same asthe petroleum in the reservoir and a boiling point below the boilingpoint of the petroleum in the reservoir.

5. A method according to claim 1 in which the injection of steam iscontinued until a substantial portion of the slug of thesteam-distillable viscous liquid buffer is recovered through theproduction well.

6. A method of recovering petroleum from subterranean reservoirs usinginjected steam in which high displacement efficiencies are obtainedcomprising:

(a) penetrating a petroleum producing reservoir with at least oneinjection well and one production well spaced apart;

(b) establishing fluid communication between each of said wells and saidreservoir;

(c) injecting through said injection well a slug of liquid,steam-'distillable butler having a viscosity range of from at least 75%to essentially that of the viscosity of the petroleum to be displaced,said slug being equal to from 5 to 10% by volume of the petroleum to bedisplaced;

(d) subsequently injecting through said injection well steam at atemperature sufiicient to effect in situ distillation of at least a partof said slug in said reservoir; and

(e) recovering displaced petroleum at said production well which isdisplaced by said slug and said steam injections until said injectedsteam breaks into said production well.

References Cited UNITED STATES PATENTS 2,867,277 1/1959 Weinaug et al1669 3,085,063 4/1963 Turbak 166-9 FOREIGN PATENTS 511,768 8/1939 GreatBritain.

CHARLES E. OCONNELL, Primary Examiner. STEPHEN I. NOVOSAD, Examiner.

1. A METHOD OF RECOVERING PETROLEUM FROM SUBTERRAMEAN RESERVOIRS USINGTHE INJECTION OF STEAM IN WHICH HIGH DISPLACEMENT EFFICIENCIES OF THEPETROLEUM ARE ACHIEVED COMPRISING: (A) PENETRATING A PETROLEUM-PRODUCINGRESERVOIR WITH AT LEAST ONE INJECTION WELL AND AT LEAST ONE PRODUCTIONWELL SPACED THEREFROM; (B) ESTABLISHING FLUID COMMUNICATION BETWEEN EACHOF SAID WELLS AND SAID PETROLEUM RESERVOIR; (C) INJECTING THROUGH SAIDINJECTION WELL AS A SLUG OF A STEAM-DISTILLABLE BUFFER WHICH IS AVISCOUS LIQUID MISCIBLE WITH AND MORE VOLATILE THAN THE PETROLEUM IN THERESERVOIR HAVING A VISCOSITY OF NOT LESS THAN 75% OF THE PETROLEUM INTHE RESERVOIR;